Set the interval from three things: how much error the process can tolerate, how fast the analyser drifts, and what a wrong reading costs. Start frequent, such as monthly, record the as-found deviation at each check, and lengthen the interval when deviations stay well inside tolerance. Shorten it for safety or quality-critical measurements, or when a quality system sets the interval.
- Identify the three inputs to an interval
- Use as-found deviations to adjust the interval
- Know when to calibrate more often
- Document the reasoning
Three inputs
- Tolerance. How far from the true value can the reading be before the process or the product suffers? Tight tolerance needs frequent checks.
- Drift rate. How much does this analyser, on this sample, move per week or month? Datasheet drift figures are a start; your own records are better: Accuracy, precision, repeatability and drift in gas analysers.
- Consequence. What happens if the reading is wrong for a week? A safety trip or a product release is worth more care than a trend display.
A method that learns from the instrument
- Start with a conservative interval, for example monthly, for a new installation.
- At each check, record the as-found deviation before any adjustment.
- If deviations stay below about one third of the tolerance for several checks in a row, extend the interval, say to two months, then quarterly.
- If a deviation approaches or exceeds the tolerance, shorten the interval and look for a cause.
- Review at least yearly and after any change to the process, the sample system or the sensor.
Calibrate when the data say so, not when the calendar says so. But never let the interval grow past what you can defend if the reading is challenged.
Between calibrations: cheap checks
- A daily look at flow, pressure and condensate catches sample faults.
- A regular zero check, which is cheap, finds drift early.
- Comparing with a portable or laboratory reading gives an independent view: Online vs laboratory vs portable gas analysis.
- An automatic zero or span check on a timer, where the analyser supports it: Auto-Calibration and validation checks for gas analysers.
Safety and regulated measurements
Where the analyser protects people or releases product, interval and records are often set by the safety assessment or the quality system, and you may also need to prove it with documents: Records, audit trails and qualification for regulated plants. Write down the reasoning, not just the number.
Sensor life and calibration
A calibration cannot restore a worn sensor. If span needs large adjustment at each check, or the response is slow, the sensor may be near the end of life: Sensor life, poisoning and replacement in gas analysers.
- Interval depends on tolerance, drift rate and the cost of a wrong reading.
- Start frequent and use as-found deviations to extend or shorten.
- Cheap daily checks and zero checks fill the gaps.
- Document the reasoning, especially for safety and quality.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
What information from each check lets you adjust the interval?
The as-found deviation, recorded before any adjustment.
When should you shorten the interval?
When deviations approach the tolerance, or for safety and quality-critical measurements.
Can calibration fix a worn sensor?
No. Large corrections at each check suggest the sensor is near the end of its life.
Key terms
Frequently asked questions
How often should an oxygen analyser be calibrated?
Start monthly and adjust with as-found data; safety and purity measurements may justify more often.
Is annual calibration enough?
Only if drift records show it, and the consequence of error is small.
Does auto-calibration replace manual calibration?
It can reduce manual work, but you still verify with certified gas and review the results.
Who sets the interval?
You do, from your tolerance and records, unless a standard or quality system specifies one.
